Magnesium sheets and steel sheets plated with a magnesium alloy such as Zn—Al—Mg, Zn—Al—Mg—Si, Zn—Mg, Mg/Zn, Al—Mg, or Al—Mg—Si have high corrosion resistance, but are easily blackened on the surface thereof by oxygen and moisture contained in air while being transported or when in storage.
Such blackening occurs because magnesium and zinc contained in the surface of a plated steel sheet containing magnesium is converted into composite hydroxides or oxides by contact with moisture. Blackening deteriorates the appearance of products and significantly facilitates cracking in machined portions because of a hardness difference between magnesium and zinc, thereby lowering workability.
To address these problems, methods of forming an oil coating, an anodic coating, or an organic coating on a magnesium steel sheet or a magnesium alloy plated steel sheet have been widely used. However, the oiling method is not suitable to improve the blackening resistance of such steel sheets in the case of storing the steel sheets for a long period of time. In addition, the method of forming an anodic coating makes it difficult to guarantee safety because of the use of large amounts of strong inorganic acids, hazardous to the environment and human health, and is not suitable for continuous production lines because of complex and time-consuming processes. The method of forming an organic coating requires a high-temperature condition for forming an organic coating and a long process time for drying the organic coating.
As a surface treatment technique, Korean Patent Application Laid-open Publication No. 2009-0017868 an anodic coating treatment method including ten steps for the surface treatment of a magnesium alloy product. However, the method disclosed in the patent document is a surface treatment method for a part but not for a steel sheet, and is uneconomical because of many process steps and long process times. U.S. Pat. No. 7,754,799 discloses a method of improving the chemical resistance of a magnesium alloy steel sheet using zinc oxide treated with an organic polysiloxane, a non-reactive silicone oil, and silane. However, this method also requires a long drying time even though the process temperature of drying is low, and organic polysiloxane and silicone oil decrease the adhesion of a top coat of paint. Thus, if this method is applied to magnesium steel sheets or alloy steel sheets, it is difficult to use the steel sheets in various applications. Japanese Patent Application Laid-open Publication No. H9-241828 discloses a technique for preventing blackening in a surface region of a Zn—Mg plating layer by dipping the Zn—Mg plating layer into a phosphoric acid pickling bath containing phosphoric acid in an amount of 0.01 wt % to 30 wt % to reduce the content of magnesium in the surface region of the Zn—Mg plating layer. However, the luster and appearance of Zn—Mg plated steel sheets deteriorate as a result of a pickling process, and sludge may have to be removed from a picked region.